In the employment of containers of thermoplastic material, it is necessary, in many physical applications, to ensure that the container--and thereby its plastic material--can tolerate both low and high temperatures without undesired permanent deformation. Similarly, there are demands that the container, after sealing, can withstand relatively large pressure differences between the inner space of the container and the space outside the container, without such pressure difference producing an undesired deformation of the container. The term "deformation" encompasses both the deformation maintained by the pressure differences and that deformation which remains once the pressure differences have been equalized. Wishes have also been expressed on the part of packers and fillers to be able to seal the container as soon as possible--and preferably immediately--after the container has been filled with its contents. In many physical applications, in particular when long-term storage of foods is a consideration, there are requirements that the sealing be carried out such that the connection between the sealing member, for example the lid, and the opening portion of the container be continuous and tight in all of its parts. In such instance, the connection between the sealing member and the opening portion generally consists of a loop which is closed in the circumferential direction. A further desire which has been expressed in this art is that, when the goods are of a more or less liquid nature, the filled and sealed container should have a head space (the space between the sealing member, or lid, of the container and the upper defining surface of the goods) which is as small as possible.
Hence, there are express wishes in the art for a container of plastic material which is suited for high-temperature applications and/or for the storage of liquids under pressure, for example the storage of soft drinks, beer etc. The term high-temperature applications is taken to mean, for example, that the contents in filled and sealed containers are pasteurized (60.degree.-70.degree. C.), that liquid is filled directly into the containers from boiling (hot filling), or that the contents of filled and sealed containers are sterilized (at least 121.degree. C.). It will be obvious to those skilled in the art that, during the heating cycle and cooling cycle, respectively, which take place in connection with pasteurization, hot filling and sterilization, the plastic material is exposed to extreme thermo stresses and/or that considerable pressure differences occur between the interior of the container and its surroundings atmosphere.
In general, it is required that once a container has been supplied with goods and sealed, the goods fill as large a portion of the filling space as possible. In addition, when the goods are of a more or less fluid nature, it is required that the space between the upper defining surface of the goods and the edge of the opening is large so as to minimize the risk that the contents of the as yet unsealed container overflow the edge. It is obvious, that, in case the container is of can form and has a large opening area, both of the above-disclosed requirements are mutually conflicting. In particular in physical applications in which the container, after filling, is moved to a sealing station, the requirement of a large distance between the upper defining surface of the goods and the opening edge is accentuated even further.
A major problem in the tight sealing of a hot-filled container, before its contents have cooled, is that the pressure difference between the inner space of the container and the container's surrounding atmosphere progressively increases as the contents of the container cool, since the pressure within the container progressively diminishes. As a rule, the walls of the container body consist of monoaxially or biaxially oriented material, which implies that the walls of the container will have caved-in or dimpled portions if the container is sealed relatively soon after the filling goods have been supplied to the container.